Device for the optical chopping of a laser beam

ABSTRACT

Apparatus for optical chopping of a laser beam into discrete light pulses consisting of a source of laser light, a rotating mirror, a first collecting lens located between the laser source and the rotating mirror, a first system of collecting lenses for receiving light reflected from the mirror, and a second system of collecting lenses which receive light from the first system and direct the light energy in a predetermined pattern on the surface of a substrate such as a moving web of paper to be perforated or divided.

BACKGROUND OF THE INVENTION

This invention concerns a device for optical chopping of a laser beaminto discrete light pulses incident sequentially and repeatedly on anumber of target points.

With the use of lasers for the treatment or machining of substrates, theproblem of a simultaneous or nearly simultaneous incidence on thesubstrate in several target zones often arises. The application ofseveral lasers as a rule is impossible for reasons of cost and space. Adivision of the beam by a so-called beam splitter is unfavorable becausethe resulting partial beams have a correspondingly diminished intensitywhich is not sufficient for certain machining processes and the splitbeams furthermore exhibit changing and in any case differentintensities.

If laser beams are used, e.g. for the perforation of paper, a certain,relatively high and constant intensity of the laser beam performing theperforation must be assured in order to obtain a uniform size andquality of the perforations. In that case, beam splitting would beunfavorable for the reasons mentioned above.

A device for optical chopping of a laser beam is already known in whicha continuous laser beam is introduced into an optical mirror systemconsisting of several consecutive rotating disks exhibiting reflectingand transmitting segments. The system thus generates a large number oflight paths formed by the different combination of reflecting andtransmitting segments. The beam introduced into the system is guidedinto the different light paths in steps according to the respectiveangular position of the disks and in this process, the beam issuccessively incident on different target points. Such an arrangementmakes it possible to provide a paper web passing through the system athigh speed with rows of perforations located transverse to the machinedirection. The known device has several disadvantages, however. Sinceonly a limited number of perforation combinations find space on thedisks, it is not possible to realize an arbitrarily high perforationfrequency. Since the beam paths have different lengths, this gives riseto focusing difficulties on the substrate.

This invention provides improved apparatus of the type described abovewhich will furnish the highest chopping frequency, but at the same timeis simple in structure and contains only a minimum of moving parts.

SUMMARY OF THE INVENTION

Thus, according to the invention, the laser beam, focused by a firstcollecting lens, is incident on the rotating mirror and thenintermittently sweeps a first lens system consisting of severalcollecting lenses of equal focal length. The distance of the first lenssystem to the rotating mirror corresponds to the focal length of theindividual lenses. The first system of lenses is followed by a row offocusing lenses, of which each is assigned to one of the lenses of thefirst system and which focus the parallel light beam on a substrate, forexample, a moving paper web. As long as the beam sweeps a lens of thefirst system, the intensity remains constant in the focus of therespective focusing lens. At the moment when the beam deflected by therotating mirror reaches the next lens of the first lens system, it jumpsto the focal point of the next focusing lens. The full intensity of theoriginal laser beam is available in each focusing point. If a polygonalmirror with a suitably large number of faces and high speed of rotationis used, it is possible to reach frequencies higher than thoseattainable with known devices, so that when the device is used toperforate cigarette paper, for example, the speed-determining step isthe technically realizable paper feed rather than the perforationfrequency. A particularly surprising finding in the invention is thatonly a single rotating part, i.e. the polygonal mirror, is necessary inorder to realize the extremely high chopping frequencies.

Further objects, features and advantages of the present invention willbecome apparent from a consideration of the following description, theappended claims and the accompanying drawing in which:

FIG. 1 is a schematic diagram in perspective showing the essentialfunctional components of the device; and

FIG. 2 is a schematic plan view of the beam path in the device.

According to FIG. 1, the beam originating from a laser 8 passes througha first collecting lens 10, the focus and line plane of which arelocated on a face of the rotating polygonal mirror 12 placed in the beampath behind the collecting lens 10. The polygonal mirror shown has sixfaces. The number of faces can be any desired one, however; for a givenchopping frequency, it also depends on the speed of rotation that can berealized with the polygonal mirror.

During rotation of the polygonal mirror, the beam reflected by a facewill sweep a certain angular interval; when the next face becomeseffective, the beam jumps back and sweeps the same angular interval.This interval of the beam that is reflected by the rotating polygonalmirror contains a first lens system 14, consisting of three individualcollecting lenses 14', 14", 14"'. The collecting lenses all have thesame focal length and such a position that their focus and their focalplane coincide with the focus and focal plane of the collecting lens 10on the polygonal mirror. The light exiting lenses 14' to 14"' thereforehas a parallel direction again.

In the practical version shown, the collecting lenses 10 and 14 areshown as cylindrical lenses. This has the advantage that the rotatingmirror 12 is not under such a high load by the high energy density ofthe laser beam, since cylindrical lenses are known to form not a point,but a line in the focal plane. This advantage had to be realized at theexpense of a higher precision in the positioning of the polygonalmirror, since any wobble defects of the mirror would naturally be ofgreater influence with the use of cylindrical than with sphericalcollecting lenses which image only one focus on the mirror.

The light beams exiting from the first lens system 14 are deflected by90° by deflection mirror 16 in the example shown in FIG. 1. Such adeflection may be necessary for design reasons in certain applications.It is of secondary importance for the function of the device.

The deflection mirrors 16 are followed by a further lens system 18consisting of individual spherical collecting lenses which focus theincident parallel beams on a paper web 20 located below and in the beampath behind lens system 18. The distance of lens system 18 from paperweb 20 therefore corresponds to the focal length of the individualcollecting lenses 18', 18" and 18"'. The size of the individual lensesof system 18 should preferably correspond to the size of the individuallenses of system 14, so that each lens segment of lenses 14' to 14"'corresponds to a segment of lenses 18' to 18"'. This correspondence isclearly evident in FIG. 2 in which the deflecting mirrors between lenssystems 14 and 18 have been omitted for greater clarity.

FIG. 2 shows sufficiently clearly that during a sweep of the angularinterval α, the beam exiting from lens 14' is shifted in paralleldirection by the amount X. The collecting lens 18' therefore must be ofsuch a size that it will always focus the beam in focus 22 over thetotal distance of its parallel displacement X.

In the example shown, a continuous paper web 20 is being perforated withsmall holes located in rows transverse to the machine direction of theweb. With a paper speed of e.g. 300 m/min and a required perforationspacing of 0.2 mm, the chopping frequency must amount to about 25 kHz.The chopping frequency can be adjusted almost as desired by the numberof faces of the rotating polygonal mirror 12.

If the chopping frequency is increased or the web speed decreased untilthe holes overlap, the paper web 20 is not perforated, but instead isdivided into several parallel webs. This practically results in "beamsplitting" without the disadvantages customarily inherent in beamsplitting, such as nonuniform and fluctuating intensity. In the presentcase, the total intensity of the original laser beam continues to beavailable.

What is claimed is:
 1. Device for the optical chopping of a laser beaminto discrete light impulses which sequentially and repeatedly aredirected toward a number of target points, characterized by a collectinglens through which the laser beam passes, a rotating mirror whosereflecting surface lies in the focal plane of said collecting lens, afirst system of several immediately adjacent collecting lenses arrangedin a row, which are swept by the laser beam deflected by the rotatingmirror and whose focal planes lie on the reflecting surface of themirror and a second system of collecting lenses disposed between saidfirst system of lenses and the target points, each lens of second systembeing respectively assigned a corresponding collecting lens of the firstsystem and the focal planes of the lenses of said second system being onthe target points.
 2. Device according to claim 1, characterized in thatthe mirror is a polygon mirror.
 3. Device according to claim 1 or 2,characterized in that the first-mentioned collecting lens and the lensesof said first system are cylindrical collecting lenses.
 4. Deviceaccording to claim 1 or 2 characterized in that reflection mirrors arearranged between the first system of lenses and the second system oflenses.
 5. Device according to claim 1 characterized in that the lensesof the second system are arranged in a row transverse to thetransporting direction of a substrate, the substrate being disposed atthe target points.
 6. Device according to claim 5, characterized in thatoperation of said mirror is synchronized with the transport rate of thesubstrate and the lenses of said first and second systems are soarranged and constructed that the target areas lie in adjacent rowswhich are substantially transverse to the transport direction of thesubstrate.
 7. Device according to claim 5 or 6, characterized in thatthe intensity of the laser beam is so chosen that the light impulsesduring the incidence on the substrate perforate the same.
 8. A devicefor treating a substrate by means of a laser beam comprising a rotatingmirror for reflecting a laser beam coming from a laser beam sourcethrough a system of focussing lenses arranged beside each other andwhose focal planes are on the surface of a substrate to be treated,characterized in that between the laser beam source and the rotatingmirror there is a first lens unit which focuses the laser beam on therotating mirror and in that between the rotating mirror and said systemof focussing lenses there is a further system of focussing lensesarranged beside each other and whose focal planes are also on therotating mirror so that the first-mentioned system of focussing lensesis acted upon by parallel rays of light at a constant angle ofincidence.
 9. A device as in claim 8, characterized in that the firstlens unit and the further system of focussing lenses comprisecylindrical focussing lenses.
 10. A device as in claim 8 or 9,characterized in that deflection mirrors are arranged between the twosystems of focussing lenses.